Decimate Selection down to a specific vertex count
Decimate Geometry

As your question's title - Decimate Geometry is what you're looking for. Both can find in Blender 2.79b and Blender 2.80 beta.

It collapses those selected geometry by the determined parameters, you can always redo this operator in the invoked panel to see which parameter suit for your case.

Select your target geometry in edit mode and with the mesh dropdown > Clean Up > Decimate Geometry



What if I need a beautiful edge loop?

As another answer point out, you could create a circle with desired vertices. Move it to the edge loop you want to decimate, delete the complicated one and join the circle created. And choose both the edge loop of the opened edges, use Bridge Edge Loops operator to connect them.